Sometimes you need to find out how a system performs at runtime, such as whether a given function has a certain capability or bug. If you can, make such checks when your program runs instead of when it is configured. You can check for things like the machine’s endianness when your program initializes itself.
If you really need to test for a runtime behavior while configuring,
you can write a test program to determine the result, and compile and
run it using
AC_RUN_IFELSE. Avoid running test programs if
possible, because this prevents people from configuring your package for
Run the compiler (and compilation flags) and the linker of the current language (see Language Choice) on the input, then execute the resulting program. If the program returns an exit status of 0 when executed, run shell commands action-if-true. Otherwise, run shell commands action-if-false.
The input can be made by
AC_LANG_PROGRAM and friends.
LIBS are used for linking, in addition to the
compilation flags of the current language (see Language Choice).
Additionally, action-if-true can run
for further testing.
In the action-if-false section, the failing exit status is available in the shell variable ‘$?’. This exit status might be that of a failed compilation, or it might be that of a failed program execution.
If cross-compilation mode is enabled (this is the case if either the
compiler being used does not produce executables that run on the system
configure is being run, or if the options
--host were both specified and their values are different),
then the test program is
not run. If the optional shell commands action-if-cross-compiling
are given, those commands are run instead; typically these commands
provide pessimistic defaults that allow cross-compilation to work even
if the guess was wrong. If the fourth argument is empty or omitted, but
cross-compilation is detected, then
configure prints an error
message and exits. If you want your package to be useful in a
cross-compilation scenario, you should provide a non-empty
action-if-cross-compiling clause, as well as wrap the
AC_RUN_IFELSE compilation inside an
(see Caching Results) which allows the user to override the
pessimistic default if needed.
It is customary to report unexpected failures with
autoconf prints a warning message when creating
configure each time it encounters a call to
AC_RUN_IFELSE with no action-if-cross-compiling argument
given. If you are not concerned about users configuring your package
for cross-compilation, you may ignore the warning. A few of the macros
distributed with Autoconf produce this warning message; but if this is a
problem for you, please report it as a bug, along with an appropriate
pessimistic guess to use instead.
To configure for cross-compiling you can also choose a value for those parameters based on the canonical system name (see Manual Configuration). Alternatively, set up a test results cache file with the correct values for the host system (see Caching Results).
To provide a default for calls of
AC_RUN_IFELSE that are embedded
in other macros, including a few of the ones that come with Autoconf,
you can test whether the shell variable
cross_compiling is set to
‘yes’, and then use an alternate method to get the results instead
of calling the macros.
It is also permissible to temporarily assign to
in order to force tests to behave as though they are in a
cross-compilation environment, particularly since this provides a way to
test your action-if-cross-compiling even when you are not using a
# We temporarily set cross-compile mode to force AC_COMPUTE_INT # to use the slow link-only method save_cross_compiling=$cross_compiling cross_compiling=yes AC_COMPUTE_INT([…]) cross_compiling=$save_cross_compiling
A C or C++ runtime test should be portable. See Portable C and C++.
Erlang tests must exit themselves the Erlang VM by calling the
function: the given status code is used to determine the success of the test
0) or its failure (status is different than
explained above. It must be noted that data output through the standard output
io:format/2) may be truncated when halting the VM.
Therefore, if a test must output configuration information, it is recommended
to create and to output data into the temporary file named conftest.out,
using the functions of module
conftest.out file is
automatically deleted by the
AC_RUN_IFELSE macro. For instance, a
simplified implementation of Autoconf’s
AC_INIT([LibdirTest], [1.0], [firstname.lastname@example.org]) AC_ERLANG_NEED_ERL AC_LANG(Erlang) AC_RUN_IFELSE( [AC_LANG_PROGRAM(, [dnl file:write_file("conftest.out", code:lib_dir()), halt(0)])], [echo "code:lib_dir() returned: `cat conftest.out`"], [AC_MSG_FAILURE([test Erlang program execution failed])])